1. Field of the Invention
This invention relates to industrial water cooling towers and especially an improved non-clogging film fill assembly for counterflow type towers of the type which must use water that has not been adequately treated to preclude microorganism growth or routinely filtered as may be necessary to remove foreign objects.
2. Description of the Prior Art
Industrial water cooling towers have long been used to reject heat in power generation, to provide cooling water for petrochemical processes, and serve as a means to lower the temperature of various chemical process streams and equipment. In the case of power generation plants, the cooling tower requirements are relatively large and it has been the practice in recent years to fabricate increasingly larger cooling towers. Counterflow towers have been found to be especially useful in these instances because of the efficiency of the towers and the compact nature of the structure. Cooling air may be brought into heat exchange relationship with the hot water either by way of convection through use of a natural draft stack, or by means of one or more large diameter, power-driven fans.
In order to further increase the efficiency of cooling towers for industrial applications which require the use of very large towers, efforts have been made to increase the effectiveness of heat exchange between the hot water and the cooling air. The degree of direct contact of the water to be cooled with the coolant air has a significant bearing on the efficiency of the cooling process. Counterflow towers, wherein the hot water and air are brought into countercurrent flow relationship have long been known to be efficient heat transfer units. Initial egg crate or slat splash bar towers were ultimately supplanted by film fill towers because of the greater heat transfer properties of a water film as compared with the multiplicity of droplets of water which are produced by splash fills. Furthermore, film fills are significantly shorter than splash fills thus decreasing the head on the pump delivering hot water to the tower and making operating less expensive because of the lower horsepower pump requirements.
The superior heat transfer characteristics of counterflow towers as well as improved efficiency based on lower pump heads has accelerated their use in recent years. Cooling tower designers in seeking to increase the efficiency of counterflow towers have also sought to further decrease the overall height of such towers by making the fill more effective than has been the case in the past. With the advent of synthetic resin sheets which are capable of withstanding higher temperatures without significant deformation than was previously the case, along with the development of resin formulations which are more resistant to deterioration under constant wet conditions, fill assemblies made up of sheets of the plastic for film flow of water thereover have in many instances completely supplanted prior fill structures which primarily relied upon break-up of the water for surface increase purposes instead of thin films of water over a large multiplicity of closely spaced sheets of plastic.
Although film fills have found acceptance in many applications including large industrial cooling towers for power generating plants and the like, problems have arisen by virtue of the fact that governmental regulatory agencies have imposed stricter limitations on the addition of agents to the cooling water which suppress growth of microorganisms and the like. For example, it has long been the practice to add chlorine or chlorine containing compounds to the cooling water in order to prevent microorganism growth. However, it is now known that when chlorine in high concentrations is discharged into streams or other natural bodies of water, the chlorine can produce adverse consequences which are harmful to biological life in the stream and in general increase what some deem to be undesirable pollution of the flowing water. There is some evidence that high chlorine levels in streams can actually produce dioxin-like compounds.
Cooling tower operators have routinely removed a portion of the cooling tower water in the form of blow down and returned it to the source such as a stream to prevent buildup of chemical additives in the water. As much as 10% of the water may be continuously returned to the stream or other water source as blow down. This water can contain a relatively high concentration of the additive and therefore significant amounts of chlorine, for example, may be present at the outlet of the cooling tower which discharges into the adjacent stream, lagoon, or lake water source. Concern over stream and water body pollution has led governmental authorities to restrict the use of additives such as chlorine in cooling tower water for preventing growth of microorganisms in the recirculating cooling water. In fact, absent a more acceptable anti-microbial additive than chlorine and which is available at a reasonable cost, many tower operators have elected to simply eliminate or drastically reduce the additives such as chlorine in the cooling tower water.
The result in certain instances has been catastrophic build up of microorganism growth in the flow assembly of counterflow industrial water cooling towers. One highly effective and efficient fill assembly for counterflow towers employs corrugated plastic sheets which are positioned so that adjacent corrugations cross one another at approximately a 30.degree. angle. The peaks of the corrugations therefore contact one another where the peaks cross. In a cross-corrugated fill utilizing plastic corrugated sheets which are spaced a distance such that the greatest spacing therebetween is of the order of 2/3 inch, there can be as many as 646 contact points or nodes per cubic foot of the fill assembly. These nodes serve as habitats for microorganisms which proliferate around the contact point. As the water to be cooled flows downwardly through the corrugated fill structure, microorganisms present in the water and whose growth is no longer inhibited by suitable anti-microbial compounds in the water, collect at the points of intersection of the corrugations of the fill. The microorganisms then start to multiply at the nodal points in the fill assembly. This growth can continue until complete blockage of the water flow paths through the fill unit occurs.
In like manner, unless the cooling tower water is continuously filtered, suspended solids in the make-up water from the stream or other natural water source can collect and accumulate in the water. These solids are trapped by the microorganism growths in the fill assembly and increase blockage of the water flow paths. In addition, airborne solids can build up in the water during tower operation unless the water is filtered.
The significance of the problem is apparent when it is recognized that in the case of a 500 megawatt power plant, if the plant must be shut down because of blockage of the fill assembly of the cooling tower serving such plant, the loss of revenue to the utility is many thousands of dollars per day. Replacement of the fill can take from one to two months. Thus, lost revenues readily mount to eight figure numbers.
The enormity of the problem is further demonstrated by the fact that cooling towers of the type discussed and especially those used for high-megawatt plants such a nuclear facilities, have fill assemblies whose plan area can be anywhere from one to two acres.
Another factor involves the insidious nature of the problem. Microorganism and solid object blockage of the cooling tower fill necessarily occurs in a gradual form. Thus, the performance of the tower will gradually decrease which has an adverse economic impact on cooling of the steam used in the plant and decreasing the efficiency of the generation process. This produces a slow loss in output which translates directly into decreased income to the utility. The tendency is to defer replacement of the fill for as long as possible because of the cost of tearing out the old fill and replacing it with a new assembly.